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Indoor Air Quality and Disease Prevention - Part One

Balancing the roles of centralized HVAC solutions and room-based disinfection systems

Part One: Aerosol Transmission of Disease

“Beyond any reasonable doubt”

In a July 6th “invited commentary” in Clinical InfectiousDiseases[1],239 scientists urged the CDC and WHO to acknowledge that COVID-19, the infection transmitted by the SARS-CoV-2 virus, was spread via airborne droplets. To that point, the WHO (and the CDC) had dismissed the possibility that COVID could be an airborne-transmitted infection.


In the commentary, clinicians and scholars pressed the WHO to recognize the airborne transmission route of SARS-CoV-2. They provided – and demanded attention to - the growing body of evidence which established “beyond any reasonable doubt” that the virus spreads indoors through tiny aerosols — and that the WHO’s recommendations should fundamentally change.


But the WHO wasn’t easily persuaded. In response to the July 6th commentary, Benedetta Allegranzi, the WHO’s technical lead on infection control, responded to the New York Times, with stubborn doubts about COVID and airborne transmission, “Especially in the last couple of months, we have been stating several times that we consider airborne transmission as possible but certainly not supported by solid or even clear evidence.”


To that point, six months into the unfolding pandemic, clinicians, and caregivers were advised that hand-washing and hygiene may be adequate prevention measures. The WHO and CDC positions were that viral droplets were so large that they could only land on near surfaces, and therefore not capable of longer distance contagion via aerosolization.

Wells and Riley

And now, we look back to the days of Eisenhower in 1954.


Surely, the WHO and CDC had known of William F. Wells, the “eccentric genius and originator of the droplet nucleus hypothesis... demonstrator of droplet nucleus transmission of bovine TB in rabbits.”  


The WHO and CDC had to have known that in 1954, Wells andRiley, droplet nuclei experts working at a VA hospital, proved those droplet nuclei could travel through the air, through ventilation systems, and infect others from the infectious source, many rooms away.


Their paper was published in 1962, 58 years before the COVID outbreak and the contentious discussions over the ability of droplet nuclei to travel through the air. Wells and Riley had already proven it, however, whenEisenhower was in the White House.

The Wells/Riley Infectious Equation for Respiratory Diseases

Wells and Riley also developed an equation which could help to mathematically formulate a risk assessment model for infection. The model considers many of the complex variables that influence disease transmission and infection potential.

From “Review and comparison between the Wells–Riley and dose‐response approaches to risk assessment of infectious respiratory diseases,” a number of influencing factors affect this infection process and the outcome.They are listed in Table 1. These factors add complexities to the exposure and risk assessment of pathogenic microorganisms. Many of them are not well‐understood, especially the pathogen–host interactions. As a result, statistics and probabilities are often employed to formulate quantitative infection risk, assessment models.

TABLE [1] – from “Review and comparison between theWells–Riley and dose‐response approaches to risk assessment of infectious respiratory diseases”[2]




Dispersion and distribution of airborne pathogens

How airborne pathogens disperse and distribute in the room air governs the exposure levels of the susceptible persons. The spatial distribution of airborne pathogens depends on the proximity to the infectious source, ventilation, and the geometry of the premises. The susceptible people would generally have different exposure levels and hence different degrees of infection risk. Assuming a uniform airborne pathogen distribution may cause significant error in the assessment (Noakes and Sleigh, 2008).

Ventilation strategy

Airborne pathogens can be dispersed to different locations by airflow. The ventilated airflow pattern has strong correlation to the spreading of airborne transmissible diseases (Li et al., 2007). The spatial distribution of infectious particles is very dependent on the airflow pattern. Infectious particles can be removed from the air by ventilation dilution, which depends on the ventilation rate.

Survival of pathogen

Pathogens may lose viability to cause infection by biological decay during the airborne stage, which is a sinking mechanism for respiratory pathogens. Airborne survival of pathogens often depends on the temperature and humidity (e.g., Schaffer et al., 1976).

Aerosol size

Expiratory aerosols and many other bioaerosols are polydispersed. The transport of aerosols depends on their aerodynamic size. Therefore, the dispersion of pathogen‐laden aerosols is dependent on aerodynamic size and the exposure levels to these aerosols usually have spatial variations. The deposition loss of infectious particles also depends on their aerosol size (Chao et al., 2008).

Respiratory deposition

When airborne pathogens are inhaled by the receptor organism, not all but a fraction of the inhaled pathogen‐laden aerosols may deposit on the target infection site in the respiratory tract. In addition, because of aerosol dynamics, the respiratory deposition of these aerosols is dependent on aerodynamic size. Because of the difference in respiratory deposition of aerosols with different sizes, the aerosols have different deposition fractions on different regions of the respiratory tract. For example, aerosols with sizes >6 μm are trapped increasingly on the upper respiratory tract, aerosols with sizes >20 μm generally do not deposited on the lower respiratory tract and those with sizes >10 μm generally do not reach the alveolar region (Hinds, 1999Tellier, 2006).

Heterogeneous infectivity

Different regions of the respiratory tract may have different immune mechanisms. In other words, pathogens generally have different infectivity in different regions of the respiratory tract. For example, the ID50 of influenza virus is about two orders higher when the virus was introduced to the nasal cavity by intranasal drop than introduced to lower respiratory tract via aerosol inhalation (Alford et al., 1966Douglas, 1975). As the respiratory deposition of aerosols depends on their sizes, the variation of pathogen infectivity when carried by infectious particles of different sizes was also observed, as shown by many experimental infection studies (e.g., Day and Berendt, 1972Wells, 1955).

Air turbulence

As induced by air turbulence, airborne pathogens trend to be randomly distributed in air. Any estimated exposure level or intake dose would be an expected value rather than an exact value. Air turbulence also exists in respiratory tracts. Respiratory deposition fraction of aerosols is also an expected value rather than an exact value (Hinds, 1999). In other words, when the respiratory deposition fraction of aerosols with a particular size is β, each aerosol with this size would have a probability of successful deposition equal to β.

Pathogen–host interaction

When a host organism is exposed to the pathogen, whether the organism will be infected or not depending on the infectivity of the pathogen and the immune status of the host organism (Haas et al., 1999).

Control measures

Control measures such as respiratory protection, ultraviolet irradiation and particle filtration can reduce the exposure level of the susceptible to airborne pathogens (Nazaroff et al., 1998).


The First Wave

Those who cared for the first wave of COVID patients, including physicians and nurses, were told that the COVID-19 disease was transmitted through surfaces and direct patient contact, and that there was little evidence to support transmission via an airborne vector.


When the CDC and WHO finally acknowledged the spread ofCOVID via aerosolized droplets, the pandemic was well underway.


As Wells and Riley had proven in 1954, the public and caregivers were finally told the air wasn’t safe.


Not only could coughing and sneezing transmit the disease, but it was found that even the breath of an infected person was infectious. Other aerosol reservoirs were discovered as well, including centralized, public, single, and shared plumbing systems. The aerosolization of biofilms and surface droplets. Centralized HVAC and ventilation systems.


Once the airborne vector was acknowledged, isolation, masking, contact tracing, and other PPE helped to slow the spread of the pandemic, giving the world time to develop and distribute the now historic mRNA vaccines.

The Cost to Healthcare Providers

The president of the World Health Organization acknowledged in a May 2021 address[3] that over 115,000 healthcare workers have died from COVID-19 infections worldwide.

“Speaking at the opening of theWHO's main annual assembly, Tedros Adhanom Ghebreyesus paid tribute to the work of health care professionals worldwide, noting that "many have themselves become infected, and while reporting is scant, we estimate that at least115,000 health and care workers have paid the ultimate price in the service of others."

A Transformational Movement Toward Environmental Safety

We’ve moved into an era of astounding public transformation.We’ve all become more knowledgeable about clean, safe air. We’re now thinking about whether we’re safe from infection in the workplace, the doctor’s office, the grocery store, and even at home.  Many of us no longer accept infection as inevitable. We want to be safe with coworkers, students, and within our communities.


We’ve all become more conscious of our personal space, of our surroundings, and of the potential risk in simply breathing.


Even with a vaccinated public, we’ll continue to be aware of environmental risks to our personal safety and not just about COVID.


Indoor Air Quality Is About to Take Center Stage

Our journey into clean air is just beginning.  In part two, we’ll discuss the role of centralized air handling systems (HVAC) in infection prevention.  

[1]Lidia Morawska, Donald K Milton, It Is Time toAddress Airborne Transmission of Coronavirus Disease 2019 (COVID-19), Clinical Infectious Diseases, Volume 71, Issue 9, 1 November 2020, Pages 2311–2313,


[2] Sze To GN, Chao CY. Review and comparison between the Wells-Riley and dose-response approaches to risk assessment of infectious respiratory diseases. Indoor Air. 2010;20(1):2-16.doi:10.1111/j.1600-0668.2009.00621.x